Tuesday, January 15, 2013

Was Pelagiarctos a “killer” walrus? Part 1: Sharktooth Hill Pinnipeds

As a teaser for a forthcoming paper by Morgan Churchill and
myself, I thought I’d introduce a (short) new series of posts (fewer than the
last series, I promise). As the publication is not out quite yet, I
thought I could at least give an introduction to the extinct “killer” walrus
from the Sharktooth Hill Bonebed.

The Sharktooth Hill bonebed in Kern
County, California is a
widespread horizon within the Round Mountain Silt member of the Temblor
Formation. It’s exposed near Bakersfield, California,
and is middle Miocene in age. It’s approximately 10-50 cm thick, generally
lacks calcareous invertebrate fossils, but is extraordinarily rich in teeth and
bones of sharks, bony fish, birds, sea turtles, pinnipeds, dolphins, sperm
whales, baleen whales, and occasionally sea cows, desmostylians, and
terrestrial mammals. I visited Sharktooth Hill several times as a high school
student, trying to find “local” vertebrate fossils – digging well through the
night in the trenches with tiki torches and a headlamp. At many localities
frequented by amateur fossil collectors, the bonebed is exposed on a hillside
and a large linear scar follows the position of the bonebed, dug out by
collectors removing overburden to get to the fossil layer. Amateur fossil collectors have done so much digging that a trench reminiscent of World War 1 battlefields encircles many hills in the region where the bonebed is exposed. Although some collectors will spend days at a time digging through overburden - admittedly backbreaking work - some decide to risk it and tunnel into the trench to get at the bonebed. Some collectors have paid for this tactic with their lives: on my first visit in 2002, a cross was placed at one of the localities where a collector had tunneled in about ten feet and was killed when the hillside slumped down onto him; it took the authorities several days to dig out his body. The rest of the
Round Mountain Silt is mostly barren with respect to vertebrate fossils, not only explaining the attention given by collectors to the bonebed itself –
but also suggesting a “unique” environment temporarily persisted in order to concentrate
vertebrate remains. A number of strange biologic explanations have been
offered, including red tides, extensive shark predation, and even a marine
mammal calving ground. Several authors have quite rightly scrutinized these
biologic explanations, and have suggested sedimentologic processes as a cause
(Mitchell, 1966; Prothero et al., 2008; Pyenson et al., 2009). These studies
have specifically suggested that a depositional hiatus (slowdown in the
accumulation rate of sediment) permitted marine vertebrate remains to be
concentrated on the seafloor. I have some minor taphonomic reservations, but
those are best discussed another day.

One of the Sharktooth Hill localities, wife for scale.

According to Barnes (1976), the Sharktooth Hill bonebed is
the most extensively studied and richest marine mammal locality in the eastern
North Pacific; a faunal list compiled by amateur collectors can be viewed here,
and it includes roughly 140 vertebrate taxa. Some of the species on the list
are not yet described or published (“Neotherium ernsti”, for example)
and other taxa are based on old identifications and may not be borne out in the
long run (aff. Herpetocetus). Regardless of issues pertaining to the
taxonomic identity of some fossil vertebrates, the ballpark number is probably
accurate. It’s also fairly spectacular: I recently tallied up fossil
vertebrates from the Purisima Formation, and there are roughly 70 taxa present
– still impressive as hell, but not quite as gargantuan as Sharktooth Hill.
Depending upon whose publication you look at, there are anywhere from seven
(Barnes, 1972; Barnes and Hirota, 1995) to four pinnipeds present (Deméré et
al., 2003). Papers by L.G. Barnes and colleagues list several desmatophocids,
including Allodesmus gracilis, Allodesmus kelloggi, Allodesmus
kernensis, Desmatophocine B, and Desmatophocine C in addition to the
imagotariine walruses Neotherium mirum and Pelagiarctos thomasi.
According to Deméré et al. (2003), only four taxa are present – Allodesmus
kernensis (with A. kelloggi and A. gracilis subsumed as
junior synonyms), an indeterminate desmatophocid (Desmatophocine B), and the
two walruses. While it’s nowhere near as diverse as the cetacean assemblage
from the same locality, it’s fairly comparable with other fossil pinniped
assemblages from the eastern North Pacific.

The skeleton of Allodesmus kelloggi as exposed in the field. From Mitchell (1966).

In 1980,
future chief preparator of the Los Angeles County Museum of Natural History (LACM)
discovered a curious chunk of bone with teeth at Sharktooth Hill. Several years
later, he brought it in to LACM and showed it to Dr. L. G. Barnes (colloquially
known as ‘Larry’ within the field), and insisted that it was the piece of a
snout of some extinct mammal – it even had two small holes which look like
nostrils to the uninitiated. Barnes kindly pointed out that those were mental
foramina on the “chin” end of a very large jawbone of a pinniped. Larry and
Howell enthusiastically recalled this whole story for Morgan Churchill and I
when we sat at the very same table last January, thirty or so years later
(Larry Barnes has an incredible, near photographic and certainly encyclopedic
memory of marine mammal fossil specimens). Howell Thomas donated the fossil for
study, and within a few years was hired as the Chief Preparator, and Barnes
began to study the specimen. At the time, the marine mammal assemblage was
already enormous, and the pinniped assemblage well documented by hundreds of
specimens. Most of the fossils could be assigned to the large seal-like Allodesmus,
although a single jaw described by Barnes (1972) as “Desmatophocine B” didn’t
appear to be referable. “Desmatophocine B” was probably similar to Allodesmus,
which has a long narrow skull, enormous eye sockets, single-rooted teeth, and a
relatively large body. Furthermore, we know Allodesmus retained the
ability to rotate its hindflippers forward for sea-lion like terrestrial
locomotion, and it was probably a sea-lion like underwater “flyer”. Numerous
small pinniped elements appeared to be similar to a handful of elements
described by Remington Kellogg (1931) as Neotherium mirum.

Skulls of Allodesmus (left) and Neotherium (right) roughly to scale. From

Barnes and Hirota (1995) and Kohno et al. (1995).

Neotherium was an enigma for
over 60 years, and it wasn’t until more complete remains of the early walrus Imagotaria
downsi were recovered from the Santa Margarita Sandstone near Santa
Cruz, California, that Neotherium
began to make sense. Imagotaria was a sea lion-like walrus that lived
about 9-12 million years ago – a bit younger than the 15-16 Ma Sharktooth Hill
Bonebed – and by the close of the 1970’s was known by a number of well
preserved skulls and partial skeletons from Santa Cruz County. Fossils of Neotherium,
although never as common as Allodesmus, continued to trickle in from the
bonebed and were referred to Neotherium piecemeal, one or two bones at a
time by Mitchell (1961), Mitchell and Tedford (1972) and Repenning and Tedford
(1977). By the 1980’s, Barnes had amassed a collection of nearly every skeletal
element of Neotherium, identifiable as miniature and slightly more
primitive versions of that found in Imagotaria – including partial
skulls and several mandibles (eventually a complete skull was published by
Kohno et al. 1995). Barnes has been for many years working on a monograph on Neotherium
– I’m looking forward to seeing it published.

The holotype of Pelagiarctos thomasi. From Barnes (1988).

Howell Thomas’ mystery jawbone
appeared more similar to Neotherium relative to Allodesmus, with
the exception of its comparably gigantic size as well as having a fused
intermandibular joint (mandibular symphysis) and deep grooves on the sides of
the canines. Eventually, several isolated teeth that were similar to Neotherium,
but several times larger in size – were discovered from the bonebed. Some of
these teeth even fit right in to the tooth sockets in the mandible fragment.
Barnes published the fossils in 1988 and described them as Pelagiarctos
thomasi, the species name honoring Howell Thomas. The genus name Pelagiarctos
refers to the primitive dental anatomy, as ‘arctos’ refers to bears, the traditional
sister taxon of pinnipeds (the root arctos is frequently used in pinniped genus
names – Arctocephalus, Phocarctos, Hydrarctos, Pteronarctos,
etc.), as well as the inferred pelagic ecology of the animal.

The isolated teeth referred to Pelagiarctos by Barnes (1988).

Several aspects of the anatomy of Pelagiarctos,
although based on scant material, suggested a different approach to feeding in
this fossil walrus relative to other Sharktooth Hill Pinnipeds. The teeth of Pelagiarctos
were huge – very robust canines, and postcanine teeth with multiple large
cusps and sharp crests. He likened the premolars and molars to those of modern
hyenas and extinct borophagine dogs, two groups which (by observation or inference)
crack and ingest bones, suggesting that Pelagiarctos
had dental adaptations for large bite forces related to feeding on large
prey items. Furthermore, the robust mandible and fused symphysis further
suggested high bite forces. Barnes (1988) additionally noted that Pelagiarctos
is very large and numerically rare in the Sharktooth Hill Bonebed – only known by
five teeth and a mandible fragment at the time of his study, as opposed to the
hundreds of specimens known of other pinnipeds such as Allodesmus and Neotherium.
This suggested to Barnes that Pelagiarctos
was rare in California waters during the middle Miocene, further supporting his
hypothesis that it was an apex predator (apex predators at the top of the food
chain can never be very abundant because they rely on a constant stock of
abundant prey items). Barnes further postulated that the type specimen was a
male, as it had proportionally large canines; modern and fossil pinnipeds are
sexually dimorphic, including early walruses like Neotherium, Imagotaria,
and Proneotherium. One of the canines
in the holotype is broken and polished down, suggesting the tooth had been
broken and worn down after continued use in life – damage which Barnes
attributed to male combat, which occasionally results in such damage in modern
pinnipeds. Furthermore, Barnes identified some of the fossil teeth as males
because they fit right into tooth sockets on the type specimen, and those that
didn't were of similar size.

As a result of these hypotheses,
numerous fanciful reconstructions of Pelagiarctos have been produced by
paleoartists (fanciful depictions can be seen here, here, and here). and Pelagiarctos has achieved the nickname
"killer" walrus by some enthusiasts. But what do we really know about
Pelagiarctos? Stay tuned...

Mitchell
ED, Tedford RH (1972) The Enaliarctinae: a new group of extinct aquatic Carnivora
and a consideration of the origin of the Otariidae. Bulletin of the AmericanMuseum
of Natural History 151: 203-284.

4 comments:

i once emailed you about those amateur spots. Never heard back. But no sweat, you looked like you had a million things going on.

Well that kinda sucks but science is science. Looking at the robust jaws and teeth, you could probably make the same argument for the elephant seal being a voracious hunter of other marine mammals. I guess it's back to sharks and whales being the big nasty predators.

I have this little book that i got when i was a kid on marine life. In one section it describes and account by a Russian sailing crew of a walrus attacking and eating a narwhal. And there is a paper out there about observations of walruses occasionally catching and eating Thick-billed Murres. Who knew the docile and funny looking walrus was capable of such blood lust!

I don't actually recall receiving said email - it may have been an unusually busy week and was buried and subsequently forgotten. I do know that at least one or two of my email addresses never receive a number of emails, so if I don't respond ever, wait a couple days and try again!

You're right on the money there. Science is about being honest and testing hypotheses, even if it's a really cool hypothesis. Someone could probably make the same hypothesis if you only had skulls and jaws of Steller Sea Lions - which are equally robust.

That's another point we bring up in the paper, regarding a mollusk specialist preying upon birds and even seals occasionally. But, that's the subject for a sequel to this post.

We sure do - my colleague Morgan Churchill and others have just published a paper on Cope's Rule in pinnipeds, and there's an estimate of Pontolis body size within. Here's the link:http://onlinelibrary.wiley.com/doi/10.1111/evo.12560/abstract

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About the Coastal Paleontologist

I'm a paleontologist and adjunct faculty at College of Charleston in South Carolina, with research interests in Cenozoic marine vertebrates with an emphasis on marine mammals (whales, dolphins, pinnipeds, otters, sea cows, and others), but I willingly entertain brief distractions into the worlds of marine birds, sharks, and fish. My M.S. (2011, MSU-Bozeman) focused on marine vertebrate taphonomy whilst my Ph.D. (2015, U. Otago, NZ) focused on Oligocene baleen whales from New Zealand. Current research is concerned with fossil cetaceans from South Carolina including Oligocene eomysticetids, toothed mysticetes, and archaic dolphins.